Driving means formed by induction motor and method for starting the same

ABSTRACT

An induction motor as driving apparatus includes two stators in which two rotor cores each formed in a squirrel cage type are rotatably mounted on a common rotary axis; two stators which respectively surround the two rotors and on which stator windings are respectively wound; and a switching device which supplies power by sequentially switching between one side of either the one stator winding or the other stator winding and both the sides of the two stator windings. Starting characteristics and operation characteristics of an induction motor are improved thereby enlarging applications of the induction motor, enhancing operation efficiency, and reducing power consumption.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a driving means which is formed by asingle induction motor integrally having two stators and two rotors, orwhich is formed by two induction motors for driving a common load, and amethod for starting the operation of such driving means.

(2) Description of the Related Art

Generally, there have been known a line-starting method and a star-deltaconnection method as the starting method of an induction motor fordriving a load. The line-starting method achieves a sufficient torquebut, on the other hand, a large current flows at the starting operationin the case of a large capacity motor so that the electricalinstallation cost is inevitably very large. In the case where thestar-delta starter is used, although the current at the startingoperation can be limited to be low, the starting torque is also lowered.Because the starting torque is lowered, it becomes necessary to use alarger capacity induction motor, resulting in raising the cost of theinduction motor. Here, with respect to the case where a star-deltastarter of a general kind is used, the resulting torque, current andinput characteristics are shown in FIGS. 9, 10 and 11.

Methods for starting an induction motor have been disclosed in, forexample, Japanese Patent Application Kokai Publication Nos. Sho51-104513 and Sho 52-54112, and a number of methods for starting themotor by appropriately switching the stator windings between the star-and delta connection are known. In these methods, the stator windingsare constituted by a plurality of circuits (windings) and, during thestarting operation, the starting current is controlled to be low by theswitching of these windings respectively to star-connections ordelta-connections.

Each of the above described methods enables the limiting of the startingcurrent but it remains as a star-delta starting method so that, due tothe reduction of the starting current, the starting torque is inevitablylowered. Thus, the device that employs the star-delta starting cannot beapplied other than to such loads as a load having square-law decreasingcharacteristics in which a load is smaller than the starting torque inthe star-connections for the star-delta starting so that, although themotor is an induction motor, its application is limited to and the motorcannot serve as a general purpose induction motor. Further, theinduction motor employing the above starting methods lackscharacteristics of intermediate torque which is larger than the startingtorque and which enables the continuous operation.

In a prior art example wherein there are a plurality of individualstator windings provided on a single stator-core, when power is suppliedonly to one of these stator windings, the deterioration in magnetomotiveforce waves is caused, resulting in various defects such as fluctuationsof torque characteristics, starting failure, and lowered operationefficiency, and the resulting structure cannot be used in practice.

The induction motor that is started by the line-starting method is usedmostly for a load in which a load is very small or an inertia is large,and thus it embodies a method that is dictated by the starting torque ofthe induction motor. However, the period in which the load having alarge inertia requires a large torque is up to the time when therotation speed reaches its rated speed. Once the rated rotation speed isreached, the rotation speed is unlikely to change especially because ofthe large inertia so that a smaller torque is sufficient after thestarting operation. However, despite the fact that only the small torqueis needed subsequent to the starting, the output cannot be lowered afterthe starting only because the induction motor is of the line-startingtype and, under the existing state, unnecessary power is being wasted.Thus, there is a demand for the development of a driving device by aninduction motor and a method for starting the same in which the startingcurrent is reduced during the starting operation and yet to generate alarge torque, and which enables the power-saved operation during therated operation. To meet this demand, a starting compensator operation(condorfer starting) and, recently, an inverter starting operation areavailable, but each of these involves a device with a very large cost.

The induction motor is often used by being connected to an emergencyelectric generating installation. The induction motor is then used as amotor for driving an emergency fire extinguishing installation such as apump. As to a prime mover constituting a private power generatinginstallation for emergency purposes and a generator driven by the primemover, the size of the prime mover and the capacity of the generator areselected based on a coefficient which is calculated on a load currentchange rate and an input change rate with respect to the motor ratingduring the starting operation period from the time when the motor whichbecomes the load of the electric generating installation starts to thetime when the rated operation speed is reached. Therefore, as a matterof course, a study has been made not only for reducing starting currentduring the starting operation but also for reducing the rates of loadcurrent and input variations in the star-delta switching during thestarting operation. This is because, by the reduction of the requiredcapacity of the generator and the prime mover, it is possible to reducethe installation cost.

Also, the fire extinguishing installation such as a pump which isoperated in an emergency is one in which the time required for reachingthe rated operation is made as short as possible. To this end, it isdesired that an induction motor as a driving apparatus be developed inwhich, while the load current change rate and the input change rate aregiven due consideration, the starting torque is made large and thestarting operation period is made short to the fullest extent, and yetthe structure of the apparatus be made simpler as possible. The startingmethods with which the attempt has been made to realize the desireincludes a closed star-delta starting, a reactor starting, and acondorfer starting including a special type of condorfer starting.

Where the induction motor is used in, for example, a compressor in whichthe operation is frequently started and stopped or undergoes frequentchanges between high and low loads, the induction motor repeats frequentstopping and re-starting operations and, due to the starting currentduring the re-starting operation, the induction motor suffers from anintense rise of temperature thus causing the motor to become inoperable.In a large induction motor, a star-delta starting operation has beenused in order to limit the starting current during the starting period,but the star-starting frequently fails the re-starting because of thelow starting torque, and this requires the use of an induction motor ofa higher ranking in its capacity. Also, for driving a compressor, anunload type motor is available in which the motor is constantly rotatingand unnecessary air is appropriately being exhausted but, since themotor is constantly rotated, the running cost becomes high.

For the above reasons, each of the conventional induction motors asdescribed above has been such that unnecessary power is consumed becauseof a large inertia or that, although the motor is for general purposes,a special starting device is required in order to limit the startingcurrent, and this has rendered the motor to be that for special use andno longer that for general purposes. Thus, there has been a demand forthe development of technology enabling the provision of an inductionmotor as a driving apparatus which is low in cost and which, as a motorthat can cope with a large inertia without requiring any such specialhigh cost starting device as required in general purpose motor, as amotor that can be used in a private power generating installation, andas a motor that can be used in a compressor that repeats starting andstopping operations and is differently used depending on a loaded stateand an unloaded state, can be used efficiently for any purposes whilefully exhibiting its capability as an induction motor.

The selection of an output of an ordinary general purpose inductionmotor which is constituted by a single rotor and a single stator andwhich has characteristics of the starting torque being smaller thanthose of the rated torque, is not based on the rated torque as referencebut is based on the starting torque of the induction motor withreference to the load torque and, especially when the motor isstar-delta started, the starting torque inevitably becomes small therebyrequiring a kind of motor whose output is of a higher ranking, that is,a higher capacity. This requirement is more conspicuous when the loadduring the starting operation is high. Thus, there is a demand fortechniques for developing an induction motor as a driving apparatus anda method for starting the operation of the same, which is constituted bya general purpose induction motor but which can be used in connectionwith a variety of loads without the need of selecting a kind ofinduction motor in which its output is unnecessarily ranked up.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to overcome the problemsexisting in the conventional driving means and to provide an improveddriving means in which starting characteristics and operationcharacteristics of an induction motor are improved.

According to an aspect of the invention, there is provided a method forsupplying power to an induction motor having a first power generationmeans constituted by a first rotor and a first stator which surroundsthe first rotor and on which a first stator winding is wound and asecond power generation means constituted by a second rotor and a secondstator which surrounds the second rotor and on which a second statorwinding is wound, the first and second power generation means beingconnected to a common load, the method comprising the followingsequential steps:

a first step of supplying power to the first and second stator windingsafter both the windings are respectively connected in a star-form;

a second step of supplying power to the first and second stator windingsafter a connection of the second stator winding is changed into adelta-form while a connection of the first stator winding remains as thestar-form; and

a third step of supplying power to the first and second stator windingsafter the connection of the first stator winding is changed into adelta-form while the connection of the second stator winding remains asthe delta-form.

According to another aspect of the invention, the induction motor maycomprise a first induction motor formed by the first rotor and the firststator and a second induction motor formed by the second rotor and thesecond stator.

Also, a switching device is arranged such that both the stator windingscan respectively be switched over between star connections and deltaconnections with respect to the power source.

With the above described switching device, it is possible to provide, atlow cost, means which can be used when, for example, a load has a largeinertia or it is needed to make small the magnitude of current/inputchanges during the connection switching. This is made possible either bya starting method in which firstly the power supplying is made byrespectively delta-connecting the two stator windings and, after therated rotation speed is reached, one of the stator windings isstar-connected or delta-connected and the power supplying is made to theone of the stator windings, or by a starting method in which firstly thepower supplying is made by respectively star-connecting the two statorwindings, secondly one of the stator windings is delta-connected, andthirdly the power supplying is made by respectively delta connecting thetwo stator windings.

Further, with the switching device, the starting operation can becarried out wherein the two stator windings are respectivelydelta-connected and the switching is made between the power supplying tothe two stator windings and the power supplying to one of them. This canbe effectively applied to a load which repeats the rotation and stoppingoperations or a load which changes between a high load and a low load.

First, since the induction motor as the driving apparatus according tothe invention comprises two squirrel cage type rotors which arerotatable on a common rotary axis, two stators which respectivelysurround the two rotors and on which stator windings are wound, and aswitching device which supplies power by sequentially switching betweenone and the other of the stator windings or switching to one of thestator windings, it is possible to use this induction motor as a generalpurpose induction motor, which can be operated in a power saving mannerafter a line-starting operation, or can make a star-delta startingoperation. Moreover, since there are provided two sets of the statorsand two sets of the rotors, the supplying power only to one of the setsthereof enables the continuous operation with an intermediate torque.This has not readily been realized by the conventional induction motor.

The induction motor structure as the driving apparatus according to theinvention comprises two induction motors which are connected for drivinga common load and a switching device which supplies power bysequentially switching between stator windings of one of the twoinduction motors and stator windings of the other of the two inductionmotors, or switching to the stator windings of either of the twoinduction motors. Thus, not only is it possible to use each of themotors, as it is, as general purpose induction motor, it is possible tomake the starting with one of the induction motors as a line-startingoperation, in which case the starting torque of this one induction motorbecomes larger than when an induction motor having the same total outputas two of such induction motors is star-delta connected for the startingoperation. Also, in no cases will the starting current exceed the ratedcurrent as being the total of the two induction motors.

The switching device is so arranged that both the stator windings canswitched over between the star-connections and the delta-connectionswith respect to the power source, and this enables the induction motoras the driving device to cope with all kinds of load torques. Moreover,this switching device can be constructed using a simple switch having acapacity about half that of a conventional switch, with the number ofsuch switches being only two to five and also the rating being half thatof the conventional switch, thus enabling the realization of theswitching device at a low cost. Other functions, for example, theswitching control of the switching device which is made by sequentialswitching based on such values as the number of revolutions, timeperiods and load torques, can be realized by conventional techniques.

The starting operation method using the switching device, in whichfirstly two stator windings are parallel delta-connected and the poweris supplied to both the stator windings and, after the rated number ofrevolutions is reached, one of the stator windings is star-connected ordelta-connected with the power supplying being made only to the one ofthe stator windings, is suited to the starting operation and thesubsequent operation when the load has a large inertia. A machine suchas a press or a crusher requires a large torque for the startingoperation, but it does not require a large torque any longer once thespeed has reached the rated number of revolutions. Thus, in theinduction motors according to the invention, after the rated number ofrevolutions has been reached, one of the stator windings isdelta-connected by the switching device and the power supplying is madeonly to the one of the stator windings for continuous operation and, inthis way, it is possible to carry out the operation with the powerconsumption being greatly reduced to below half the normal powerconsumption. That is, the reduction is made to about a half both in thecurrent and the torque. However, the starting torque in this state islarger than the torque under the parallel star connections. Also, wherethe induction motor according to the invention is used in the startingoperation of a crusher which has a speed reducer, since the torque iscomparative low during the starting operation, it is possible for one ofthe stator windings to be delta-connected with the power supplying beingmade only to the one of the stator windings and, when the operation isfor a comparatively large load, both the stator windings areparallel-delta-connected with the power supplying being made thereto,thus making it possible to draw out a large torque.

The method for the starting of operation in which the switching deviceoperates such that, firstly the two stator windings areparallel-star-connected with the power supplying being made to both thestator windings, secondly either one of the stator windings isdelta-connected with the power supplying being made only to the one ofthe stator windings, and thirdly the two stator windings areparallel-delta-connected with the power supplying being made to both thestator windings, is best suited to the operation of a pump or a fan.

The load in the above is a square-law decreasing load in which thegradual increase of the load is slow during the starting operation butthe load becomes larger as the rated operation is approached.Conventionally, in such a starting operation, the switching is made fromthe star-connections to the delta-connections. The present inventionfollows the same switching pattern but, since the two sets of thestators and the rotors are provided, the two sets of the stators and thetwo sets of the rotors effectively function independently so that thestarting torques during the starting operation period undergo ratiochanges of 1/3, 1/2 and 1, whereby a soft start is realized and thestarting operation characteristics are enhanced even with the reductionof, for example, load current and changing ratios of input.

The method for the starting of operation in which the switching deviceoperates such that, firstly the two stator windings areparallel-star-connected with the power supplying being made to both thestator windings, secondly either one of the stator windings isdelta-connected, and thirdly the two stator windings areparallel-delta-connected, is best suited to a square-law decreasing loadsuch as a disaster prevention pump or a disaster prevention fan whichbecomes a load of a private power generating installation.

That is, when the starting is made by the parallel-star-connections, theratio is 1/3 both in the current and the torque as compared with that ofthe torque under the parallel-delta connections. The operation beginswith the parallel-star-connections and, at the intersection with theload torque, one of the stator windings is delta-connected, followed bythe switching to the delta-connections and the star-connections inparallel. At this switching, the current of the independentdelta-connections is 1/2 that of the parallel delta-connections and thecurrent of the independent star-connections is 1/6 that of theparallel-delta-connections so that the current ratio changes from 1/3 to4/6. Further, the operation takes place by the parallel-star-deltaconnections and, at the intersection with the load torque, the switchingis made to the next parallel-star-delta connections. At this time, thecurrent ratio changes from 4/6 to 1.

The current ratio change at the first switching is from 1/3 to 4/6, andthis is incomparably small when compared with a large change of 1/3 to 1during the conventional star-delta switch starting operation. Thecurrent change to take place at the next switching is from 4/6 to 1.When this switching takes place, the number of revolution has alreadyapproached close to the rated revolutions and the load current has beenreduced to a small value, so that any influence this change from 4/6 to1 may bring about is extremely small as compared with that in the priorart. Similarly, the change in the input is reduced to an extremely smallvalue as compared with that in the conventional star-delta switching.This is accountable, as already explained, by the fact that, since thetwo sets of the stators and the rotors are provided, the inductionmotors made up of the respective sets thereof effectively function toproduce a consolidated torque. Moreover, assuming that each of them isconstructed by a motor having the same capacity, the torque of each ofthem is 1/2 that in the conventional example, so that the capacity ofthe electric circuits to be incorporated therein can be smaller and thecost is lower accordingly, thus offering advantages in both quality andcost aspects.

The method for the starting of operation in which the switching deviceis such that the two stator windings are respectively delta-connectedwith the power supplying being made switchably between both the twostator windings and one of the two stator windings, is best suited to acompressor in which the change of load is repeated between high and low.In the compressor, the operation and the stopping are repeated, or thehigh and low loads in the continuous operation are repeated.

Although a problem is not great in a small size compressor, when thesize of the compressor increases, it requires a large size motorbecause, with the star-delta connections, the starting torque is notsufficient for the repetition of operation and stopping and, with theline-starting operation, such a repetition cannot be tolerated.Therefore, with the induction motor having a large torquecharacteristics (with two stator windings) and a half of the torquecharacteristics (with one of the stator windings), when the large torqueis used only momentarily for the starting operation and is immediatelyswitched to the half torque characteristics, it is possible to make aquick reduction of the large starting current to about a half thereofand, as a consequence, the temperature rise of the motor is smaller thanthat in the conventional line-starting operation alone.

Where a high load and a low load are repeated in the continuousoperation, the conventional motor is operated with the same torque bothwhen the load is high and low so that, irrespective of the low load, thepower consumption is large. Among the prior art motors, there is a motorwhich is operated by using star-delta starting with the star-connectionstaking place while the load is low. However, in the star-delta switchingused therein, since there is a large difference between the torqueduring the star-connection and the torque during the delta-connectionand the stator windings are once totally cut from the power source,there arises a maintenance problem such as wearing of contacts.According to the present invention, since the motor used is an inductionmotor which has large torque and half torque characteristics and whichis operated with the large torque (power being supplied to the twostator windings) when the load is high and with the half torquecharacteristics (power being supplied to one of the two stator windings)when the load is low, it is possible to carry out the power-savedoperation when the load is low. Furthermore, for the switching betweenthe large torque and the half torque, since there occurs no such problemas the disconnection of one of the stator windings from the power sourceand also since the current that flows to the other of the statorwindings is about half the large torque, it is possible to enhance thereliability in the maintenance aspects such as the wearing of contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention explained with reference to theaccompanying drawings, in which:

FIG. 1 is a sectional view of an induction motor as a driving meansaccording to the invention;

FIGS. 2A-2C are structural diagrams of another induction motor as adriving means according to the invention;

FIG. 3 is a diagram showing connections effective when a load having alarge inertia is driven;

FIG. 4 is a graph showing torque characteristics obtained by therespective connections according to the invention;

FIG. 5 is a graph showing current characteristics obtained by therespective connections according to the invention;

FIG. 6 is a graph showing input characteristics obtained by therespective connections according to the invention;

FIG. 7 is a diagram showing connections according to the invention thatare effective when a load is a pump or fan used in an emergency privatepower generating installation;

FIG. 8 is a diagram showing connections according to the invention thatare effective when a load is one in which operation and stopping arerepeated;

FIG. 9 is graph showing torque characteristics obtained by thestar-delta switching in a conventional induction motor;

FIG. 10 is a graph showing current characteristics obtained by thestar-delta switching in a conventional induction motor; and

FIG. 11 is a graph showing input characteristics obtained by thestar-delta switching in a conventional induction motor.

PREFERRED EMBODIMENTS OF THE INVENTION

A first preferred embodiment of the invention is explained withreference to FIG. 1. First the construction of an induction motoraccording to the invention is explained. In the induction motorrepresented by the numeral 1, two rotor cores 3 and 4 are axiallymounted on a rotary shaft 2 with a predetermined space being providedtherebetween, rotor conductors 5 and 6 are provided respectively on therotor cores 3 and 4 and, at two sides of the rotor cores 3 and 4, therotor conductors 5 and 6 are formed in a squirrel cage form by beingshort-circuited by short-circuiting rings 7 and 8, thereby forming aunitary rotor structure 9 having the two rotors. Further, stators 14 and15 comprise stator cores 10 and 11 that are provided so as to surroundthe respective rotor cores 3 and 4 with predetermined spaces beingprovided therebetween and stator windings 12 and 13 that are woundrespectively on the stator cores 10 and 11.

On the two sides of the machine frame 16 on which the stators 14 and 15are provided, there are provided a bearing 19 containing bearing balls17 and a bearing 20 containing bearing balls 18 and, due to thesebearing balls, the shaft 2 of the rotor structure 9 that is axiallysupported rotates coaxially with the stators 14 and 15. Also, the statorwindings 12 and 13 of the induction motor are connected to a switchingdevice 25, and the induction motor forms a driving apparatus. Accordingto the invention, in order to realize a compact induction motor, tworotors and two stators are provided in parallel in one machine frame,which can also be handled at an installation site in the same manner asfor the conventional induction motor.

A second preferred embodiment of the invention is explained withreference to FIG. 2. FIG. 2 shows an induction motor structure as adriving apparatus in which two general purpose induction motors are usedfor driving a common load. FIG. 2A shows an arrangement in which theinduction motors 23 and 24 are coupled to a load 22 from its two sidesthrough respective coupling members 27, and the induction motors 23 and24 are electrically connected to a power source 26 through a switchingdevice 25.

FIG. 2B shows an arrangement in which the induction motor 23 drives theload 22 through the coupling members 27, and the induction motor 24drives the load through a pulley 28 and a belt 29. The connectionbetween the induction motors 23 and 24 and the power source 26 is thesame as that explained with reference to FIG. 2A.

FIG. 2C shows an arrangement in which both the induction motor 23 andthe induction motor 24 are coupled to the load 22 for driving it throughthe pulley 28 and the belt 29. Here, the connection between theinduction motors 23 and 24 and the power source 26 is also the same asthat explained with reference to FIG. 2A.

In the above cases, the motors required are two general purposeinduction motors each of which may only have half the capacity that isrequired for the load. Thus, the cost of the induction motors, that is,the total cost of the two induction motors each having half the requiredcapacity, will not exceed the cost of the induction motor having therequired capacity.

Now, FIG. 3 is referred to for explaining the switching device 25 whichis constructed as a unit with the induction motor 1. In controlling theswitching in this switching device, the effective controlling methodsinclude those in which, by detecting changes due to the load to theinduction motor in any of or in combination of values of revolutions,load currents, temperatures and times, the switching is made based onthe detected values. Where the switching device employs a simplesequence and use is made of, for example, only the times or therevolutions for the switching operation, the handling of the switchingdevice will be exactly the same as that for the prior art inductionmotor. It is also possible to use the switching device in combinationwith other control devices or control boards (not shown) for thecontrolling of the switching device.

The stator windings 12 and 13 in the switching device 25 and variousconnections between the stator windings 12 and 13 and the switches S areshown in FIG. 3 and FIGS. 7 and 8. Changes in torques and currents dueto the switching are shown in FIGS. 4 to 6. Since the two inductionmotors 23 and 24 and the switching device 25 shown in FIG. 2 are thesame as those in the first embodiment of the invention, the explanationto follow hereunder is omitted for the second embodiment.

With reference to FIG. 3, one side of the 3-phase stator windings 12 isconnected to power sources R, S and T through a main switch S1 while theother side of the 3-phase stator windings 12 is connected to a switch S2for star-connections. The 3-phase stator windings 13 are connected inparallel with the stator windings 12 through a switch S3. Also, thestator windings 12 and 13 are both delta-connected at a switch S4. Ifthe respective switches S are controlled by separate sequence circuits(not shown), it will be possible to automate the switching of theswitches during the starting operation based on, for example, times orrevolutions.

The above explained arrangement enables the parallel-delta-connections,parallel star-connections, single delta-connections and singlestar-connections. Excepting for single star-connections in which atorque is very small, three types of characteristics, that is, fortorque, current and input characteristics are shown in graphs of FIGS.4, 5 and 6. It will be seen therefrom that the arrangement can beapplied to any loads in the same way as the prior art induction motorand that, of course, the line-starting operation is also possible. Theinduction motor as the driving apparatus according to the invention canbe applied to any load torque, even to a torque with a large inertia ora torque with square-law decreasing characteristics, only by changing aswitching sequence in the switching device. Moreover, since there areprovided two sets of stators and two sets of rotors, if the powersupplying is made only to one side of the 3-phase stator windings by thedelta-connections, the motor can be started by an intermediate torque(single delta-connections in FIG. 4) which is, as shown in FIG. 4 by thesingle delta-connections, larger than the star-delta started torque ofthe prior art or the rated torque. Also, the starting current (singledelta-connections) is so low as being about 1.5 times that of theparallel-star-connections and, moreover, continuous operation ispossible. These characteristics are the ones which have not readily beenachieved or realized by the prior art induction motors. Theparallel-star-connections or the parallel delta-connections according tothe invention are different from the connections in the prior art whichare parallel connections of two circuits within one stator and, sincethey are independent star-connections and delta-connections of theinduction motors each having substantially half the rated output, thereis no interference in magnetic circuits arranged in parallel, or thereis no influence to the changes in the magnetic characteristics andtorque characteristics caused by the power supplying only to one of thestator windings. Moreover, the switches S2, S3, S4 are, with theexception of the main switch S1, switches each having a capacity halfthat of prior art switches, and also the number of these switches canonly be two to five (which is also true in other embodiments) and suchswitches can be realized at a low cost.

Now, the explanation is made for the case wherein a load having a largeinertia is started and operated by the induction motor having theconnections as shown in FIG. 3. In this case, in the state in which allthe switches are opened, the switching operates such that firstly theswitches S3 and S4 are closed and the two 3-phase stator windings 12 and13 are parallel-delta-connected. Then, the power supplying is made tothe two 3-phase stator windings 12 and 13 and, when the rated number ofrevolutions is reached, the switch S3 is opened and only one 3-phasestator winding 12 is delta-connected (single delta) and the powersupplying is made only to this one 3-phase stator winding. The torque,current and input characteristics obtained in the respective connectionsare shown in FIGS. 4, 5 and 6.

A load having a large inertia as in presses or crushers requires a largetorque for the starting operation, but such a large torque is no longerrequired once the rated number of revolutions is reached. Thus, byclosing the switches S3 and S4, the 3-phase stator windings 12 and 13are started by the parallel-delta connections as shown in FIG. 3 and,after the rated number of revolutions is reached, the induction motoroperates such that, by opening the switch S3 by the switching device 25,only one 3-phase stator winding 12 is delta-connected and is switched tothe continuous operation (single delta-connections in FIG. 4) with thepower supplying being made only to this one 3-phase stator winding. Inthis way, during the operation, it is possible to save the power tolower than the ordinary rated current. That is, both the current and thetorque can be reduced to about a half. However, the starting operationtorque is increased to a torque larger than the rated torque obtained bythe parallel-delta connections.

Contrary to the above, in the starting operation of the crusher in whicha speed reducing device is used, the starting torque is comparativelylow so that the starting can be made by closing the switch S4 anddelta-connecting one 3-phase stator winding 12 with the power supplyingbeing made only to this one 3-phase stator winding (singledelta-connections in FIG. 4). For the operation of a comparatively largeload, both the 3-phase stator windings 12 and 13 are delta-connected(parallel-delta connections in FIG. 4) with the power supplying beingmade thereto and, in this way, a large torque can be obtained.

Now, the explanation is made for the case wherein a pump or fan which isa square-law decreasing load is started by the induction motor as adriving means, in which the connections are made as shown in FIG. 3. Inthis case, in the state in which all the switches are opened, theswitching operates such that firstly the switches S3 and S4 are closedand the two stator windings 12 and 13 are parallel-delta-connected withthe power supplying being made thereto by closing the switch S1.Secondly, the switches S2 and S3 are opened, the switch S4 is closed andone stator winding 12 is delta-connected with the power supplying beingmade to this one stator winding 12. Thirdly, the switch S3 is closed andthe two stator windings 12 and 13 are parallel-delta-connected and thepower supplying is made to both the stator windings 12 and 13.

A pump or a fan is a square-law decreasing load in which the gradualincrease of the load is slow during the starting operation but the loadbecomes large as the rated operation is approached. Conventionally, insuch a starting operation, the switching is made from thestar-connections to the delta-connections. The present invention followsthe same switching pattern but, since the two sets of the stators andthe rotors are provided, there is a large difference from the star-deltaswitching which is made by a conventional single induction motor. Thatis, the two sets of the 3-phase stator windings 12 and 13 effectivelyfunction independently so that the ratios of the currents and thetorques with respect to the ultimate parallel-delta-connections duringthe starting operation are 1/3 during the parallel-star-connectionoperation (parallel-star connections in FIG. 4), 1/2 during theindependent delta-connection operation (single delta-connections in FIG.4), 1 during the parallel-delta-connection operation(parallel-delta-connections in FIG. 4), and these changes ensure therealization of a soft start and largely improve the startingcharacteristics from those of the conventional star-delta startingoperation.

FIG. 7 shows a second example of connections in which the statorwindings 12 and 13 and the switches S are connected in the switchingdevice 25. Here, one side of the 3-phase stator windings 12 is connectedto power sources R, S and T through a main switch S1 while the otherside of the 3-phase stator windings 12 is connected to a switch S2 forstar-connections. The 3-phase stator winding 12 is delta-connectedthrough the switch S4. One side of the 3-phase stator windings 13 isconnected to the power sources R, S and T through the 3-phase statorwinding 12 through the switch S1. The other side of the 3-phase statorwinding 13 is connected to a switch S5 for star-connections. Also, the3-phase stator winding 13 is delta-connected by a switch S6. Therefore,the 3-phase stator winding 12 results in star-connections by the switchS2 and in delta-connections by the switch S4. Further, the 3-phasestator winding 13 results in star-connections by the switch S5 and indelta-connections by the switch S6.

Next, the explanation is made for the case in which, by using theinduction motor with the switching device as shown in FIG. 7, thestarting operation takes place in a square-law decreasing load such as apump or a disaster prevention fan which becomes a load of a privatepower generating installation. In this case, the starting is made by theswitching device in the following manner. In the state in which all theswitches are opened, firstly the switches S2 and S5 are closed so thattwo stator windings 12 and 13 are parallel-star-connected and then theswitch S1 is closed so that the power supply is made to the two 3-phasestator windings 12 and 13. Secondly, the switch S2 of one stator winding12 is opened and is delta-connected by closing the switch S4. Thirdly,the switch S5 of the 3-phase stator winding 13 is opened and the switchS6 is closed whereby the two 3-phase stator windings 12 and 13 areparallel-delta-connected.

The torque changes by the switching of the connections and the resultingcurrent and input characteristics are as follows.

That is, when the starting is made by the parallel-star-connections, theratio becomes 1/3 both in the current and the torque as compared withthose in the parallel-delta connections. The operation takes place inthe parallel-star-connections and, at the intersection A (FIG. 4) withthe load torque, one side 3-phase stator winding 12 is delta-connectedand is switched in parallel with the star-connections of the other side3-phase stator winding 13. At the switching at this time, the current ofthe 3-phase stator winding 12 of the independent delta-connections is1/2 of that in the parallel-delta-connections and that of the 3-phasestator winding 13 of the independent delta-connections is 1/6, and thismeans that the current changes from the ratio 1/3 to the ratio 4/6.Further, the operation takes place under theparallel-star-delta-connections and, at the intersection B with the loadtorque, the switching is made to the next parallel-delta-connections.The current then changes from the ratio 4/6 to the ratio 1.

The current change at the first intersection A is from 1/3 to 4/6. Thischange is incomparably smaller than a large change from the ratio 1/3 tothe ratio 1 in the conventional star-delta switching start operation. Atthe next intersection B, the current change is from 4/6 to 1 and, sincethis switching takes place at a point when the speed is already close tothe rated number of revolutions, the value of the load current is verysmall so that any influence that the change from the 4/6 to 1 may bringabout is very small when compared with the prior art. Also, if, by theswitching device 20, the torque by the single delta-connections alone isadded to the next torque of the parallel-star-connections, the changeratios of the input and the current can further be made smaller. Theseadvantages result because, as already explained, the two sets of 3-phasestator windings constitute two induction motors which functioneffectively as a unit for producing the necessary torque. Furthermore,if the two motors are of the equal capacity, the rated capacity of eachof them can be 1/2 that of the conventional motor, and the circuits tobe incorporated may employ those of proportionally lower capacity andcost so that there are significant advantages in both the aspects ofperformance and cost.

Finally, FIG. 8 is a connection diagram showing, as a third example, theconnections of the stator windings 12 and 13 and the switch S in theswitching device 25. That is, one side of the 3-phase stator winding 12is connected to the power sources R, S and T through the main switch S1,and the other side of the 3-phase stator winding 12 is connected to theone side of the 3-phase stator winding 12 for the delta-connections.Also, the 3-phase stator winding 13 is connected in parallel to the3-phase stator winding 12 through the switch S3, resultingsimultaneously the 3-phase stator winding 13 in the delta-connections.

Next, the explanation is made for the case wherein a load such as acompressor in which the repetition is made for operating and stopping orfor high and low is driven by the induction motors with switchingdevices as shown in FIG. 8. In this case, by the switching device 20,the starting operation is made by closing the switches S1 and S3 anddelta-connecting the 3-phase stator windings 12 and 13 and, immediatelyafter the starting, the switch S3 is opened and the power supplying isswitched only to the 3-phase stator winding 12 out of the two 3-phasestator windings 12 and 13. Where the load is high, the switch S3 isclosed and the power supplying is made to the two 3-phase statorwindings 12 and 13 and, where the load is low, the switch S3 is openedand the power supplying is made only to the one 3-phase stator winding12.

Although a problem is not significant in a small size compressor, whenthe size of the compressor increases, it requires a large size motorbecause, with the star-delta-connections, the starting torque is notsufficient for the repetition of operation and stopping and, with theline-starting operation, such a repetition cannot be tolerated.Therefore, with the induction motor having a large torque (two 3-phasestator windings 12 and 13) and a half of the torque characteristics (one3-phase stator winding 12), when the large torque is used onlymomentarily for the starting operation and is immediately switched tothe half torque characteristics (single delta-connections in FIG. 4), itis possible to make a quick reduction of the large starting current toabout a half thereof and, as a consequence, the temperature rise of themotor is smaller than that of the conventional line-starting operationalone .

Where a high load and a low load are repeated in the continuousoperation, the conventional motor is operated with the same torque bothwhen the load is high and low so that, irrespective of the low load andeven when no air is used, the power consumption is large. Among theprior art motors, there is a motor which is operated by using star-deltastarting with the star-connection taking place while the load is low.However, in the star-delta switching used therein, since there is alarge difference between the torque during the star-connection and thetorque during the delta-connection and the stator windings are oncetotally cut from the power source, there arises a maintenance problemsuch as wearing of contacts. According to the present invention, sincethe motor used is of an induction motor structure which has large torque(parallel-delta-connections) and half torque characteristics (singledelta-connections) and which is operated with the large torque when theload is high and with the half torque characteristics (power beingsupplied to one of the two stator windings) when the load is low, it ispossible to carry out the power-saved operation when the load is low.Furthermore, for the switching between the large torque and the halftorque, since there occurs no such problem as the disconnection of the3-phase stator winding 12 from the power source and also since thecurrent that flows to the other stator winding 13 is about half therated torque, it is possible to enhance the reliability in themaintenance aspects such as the wearing of contacts of the switches S.

In the above embodiment, by using, for example, the connections as shownin FIG. 7, if the switch S2 is closed with the connections being changedto single star-connections, the torque characteristics will become halfthose of the parallel star-connections shown in FIG. 4, thus providingthe torque characteristics which are better suited to the square-lawdecreasing characteristic load.

The conventional induction motor of the kind to which the presentinvention relates has been such that it consumes unnecessary power for alarge inertia or that, although the motor is for general purposes, aspecial starting device is required in order to limit the startingcurrent, thus making the motor a special use motor and no longer ageneral purpose motor. The present invention provides an induction motorwhich is low in cost and which can be used efficiently for any purposeswhile fully exhibiting its capability as an induction motor, that is, asa motor that can cope with a large inertia without requiring any suchspecial high cost starting device as required in the conventionalgeneral purpose motor, as a motor that can be used in a private powergenerating installation, and as a motor that can be used in a compressorthat repeats starting and stopping operations and is differently useddepending on a loaded state and an unloaded state. The present inventionalso provides a method for starting the operation of the inductionmotor.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeof the invention as defined by the claims.

What is claimed is:
 1. A method for supplying power to an inductionmotor having a first power generation means constituted by a first rotorand a first stator which surrounds said first rotor and on which a firststator winding is wound and a second power generation means constitutedby a second rotor and a second stator which surrounds said second rotorand on which a second stator winding is wound, said first and secondpower generation means being connected to a common load, said methodcomprising the following sequential steps:a first step of supplyingpower to said first and second stator windings after both the windingsare respectively connected in a star-form; a second step of supplyingpower to said first and second stator windings after a connection ofsaid second stator winding is changed into a delta-form while aconnection of said first stator winding remains as the star-form; and athird step of supplying power to said first and second stator windingsafter the connection of said first stator winding is changed into adelta-form while the connection of said second stator winding remains asthe delta-form.
 2. A method for supplying power to an induction motoraccording to claim 1, wherein said induction motor is a single motor inwhich said first and second rotors are mounted on a common rotary shaftwith a predetermined space being provided therebetween and said firstand second stators are surroundingly facing said first and secondrotors, respectively.
 3. A method for supplying power to an inductionmotor according to claim 1, wherein said induction motor comprises afirst induction machine formed by said first rotor and said first statorand a second induction machine formed by said second rotor and saidsecond stator.